1. Field of the Invention
The present invention relates to an apparatus and a method for ultrasonic testing which is one of non-destructive testing methods. More particularly, the present invention is related to an apparatus and a method for ultrasonic testing by using an array ultrasonic sensor.
2. Description of the Related Art
Conventional ultrasonic testing methods targeting various kinds of structural materials utilize an ultrasonic sensor composed of a single element for transmission and reception of an ultrasonic wave. The ultrasonic sensor receives an ultrasonic signal reflected by a defect or the like inside an object under test to detect a defect based on the propagation time of the ultrasonic signal and the position of the ultrasonic sensor.
Specifically, conventional methods comprises the steps of: appropriately selecting an angle and vibration mode (longitudinal wave, transversal wave, etc.) of an ultrasonic wave to be applied to the object under test; moving the ultrasonic sensor to obtain a position at which a sufficiently strong reflected wave (echo) can be obtained from a defect; and identifying the size of the defect based on a difference between reception times of reflected waves from the bottom surface (far-side boundary surface) and the top face (near-side boundary surface) of the object under test, multiplied by the sonic velocity of the material of the object under test.
These methods are commonly used for ordinary defect inspections because of their simple operating principle and relatively simple instrumentation. However, since it is necessary to measure a reflected ultrasonic wave and evaluate the existence and position of a defect only from reception time of the reflected wave, high-accuracy testing requires experienced inspector and is time-consuming.
In recent years, new ultrasonic testing methods have been developed. As represented by the phased array method, these new techniques image the inside of an object under test with high accuracy (refer to, for example, Nonpatent Document 1).
The phased array method utilizes a so-called ultrasonic array transducer composed of an array of several tens of piezoelectric elements and operates on a principle that wave fronts of ultrasonic waves transmitted from the piezoelectric elements mutually interfere to form one combined wave front in the course of propagation. Therefore, controlling the timing of ultrasonic wave transmission from each piezoelectric element with a time delay (on a time-shift basis) makes it possible to control the ultrasonic beam angle and allow the ultrasonic wave to focus.
When receiving reflected ultrasonic waves, summing up these waves received by the piezoelectric elements on a time-shift basis makes it possible to control the receive beam angle of one combined ultrasonic wave as well as receive ultrasonic waves at one focal position in a similar way to transmission.
Generally known processes for the phased array method include the linear scanning process which linearly feeds piezoelectric elements and the sectorial scanning process which changes ultrasonic-wave transmit and receive directions in a fan-like form. Both processes can apply ultrasonic waves at high speed without moving the ultrasonic sensor and control the beam angle and focal depth position of the ultrasonic wave without replacing the ultrasonic sensor. Therefore, it can be said that both techniques enable high-speed and high-accuracy testing.
Of the above-mentioned conventional techniques, the phased array method has the advantage of controlling the beam angle and focal position of the combined ultrasonic wave by using a plurality of piezoelectric elements, and allowing high-speed and high-accuracy testing.
On the other hand, the focal depth is determined by an aperture of the array transducer (nearly equals the size of a piezoelectric element composing the array transducer multiplied by the number of elements). Therefore, testing an object having a long propagation path therein or a thick plate requires a large-sized array transducer (an array transducer composed of a number of elements) having a focal depth suitable for its size.
For example, suppose a case where a steel material (with a sonic velocity of longitudinal ultrasonic wave of 6000 m/s and a wavelength of 3 mm) is tested by using an array transducer with a frequency of 2 MHz. Generally, with an ultrasonic transducer having an aperture size of A (mm), the ultrasonic wave is strong in the vicinity of the near-sound-field limit distance (NF) represented by the formula (1) below. Therefore, an ultrasonic transducer having a larger aperture is required to test a thicker material. When using an array transducer normally having a constant frequency and a constant interval between piezoelectric elements, it is necessary to use a multi-element array transducer having increased number of piezoelectric elements for testing.
[Formula 1]NF=A2/4λ  (1)
An ultrasonic testing apparatus employing a multi-element array transducer needs to have pulser, receiver, and wiring circuits corresponding to the total number of elements in order to drive the multi-element array transducer. Accordingly, there has been a problem that a remarkable increase in size and complexity in internal structure and wiring causes degradation in portability, installability, and maintainability.
In order to solve this problem, an imaging method using a small number of pulsers and receivers corresponding to the number of some elements of the ultrasonic array transducer, for example, the synthetic aperture method is used (Nonpatent Document 2).
With the synthetic aperture method, a single ultrasonic transducer having a small sensor aperture transmits an ultrasonic wave so that it widely spreads out into an object under test, and the same or different ultrasonic transducer receives a reflected ultrasonic wave signal (echo) from the inside of the object.
The operating principle of the synthetic aperture method is that, since the propagation path of ultrasonic wave is known, a defect serving as a sound source of a received reflected ultrasonic wave exists on a circular arc having the position of a piezoelectric element which transmitted and received an ultrasonic wave as a center and the propagation distance of the reflected ultrasonic wave as a radius. (When different piezoelectric elements are used for transmission and reception, a defect exists on an ellipse arc having each of the piezoelectric element for transmission and the piezoelectric element for reception as a focal position.)
Based on this operating principle, the ultrasonic sensor transmits and receives ultrasonic waves while sequentially changing the position of an active ultrasonic transducer for transmission and reception. At each transducer position, a receive signal is spread out in a circular arc form (or in an ellipse arc form) through computer operations. Then, intersections of these circular arcs focus at one position where a defect exists (a true reflection source position) thus allowing the defect position to be located and imaged.
[Nonpatent Document 1]
Yoshikazu Yokono, Global Trend of Phased Array Ultrasonic Testing Its Practical Application and Standardization, The Japanese Society for Non-destructive Inspection, Vol. 56, No. 10, 2007.
[Nonpatent Document 2]
Michimasa Kondo, Yoshimasa Ohashi, and Akio Jitsumori, Digital Signal Processing Series Vol. 12, Digital Signal Processing in Measurement and Sensors, pp. 143-186, May 20, 1993, SHOKODO CO., LTD.
A conventional method for testing a defect of an object under test such as a structural material transmits an ultrasonic wave by using a single ultrasonic sensor and receives echoes reflected by a defect or the like inside the object under test by using a single ultrasonic sensor to detect a defect based on the propagation time of the ultrasonic wave and the position of the ultrasonic sensor. The conventional method also moves the ultrasonic sensor to obtain a position where a reflected echo from a defect is obtained, and identifies the size of the defect based on a difference between reception times of reflected echoes from the bottom and surface, multiplied by the sonic velocity of the material of the object under test. This method is commonly used for ordinary defect inspections because of its simple operating principle and relatively simple instrumentation. However, since it is necessary to measure reflected ultrasonic echoes and evaluate existence and position of a defect from the reception time of the reflected echoes, high-accuracy testing requires experienced inspector and is time-consuming.
In recent years, new ultrasonic testing methods have been developed. As represented by well-known phased array method and synthetic aperture focusing method, these new techniques image the inside of an object under test with high accuracy. The phased array method utilizes an array of a plurality of piezoelectric elements and operates on a principle that wave fronts of ultrasonic signals transmitted from the piezoelectric elements mutually interfere to form one combined wave front in the course of propagation. Therefore, controlling the timing of ultrasonic wave transmission from each piezoelectric element with a time delay (on a time-shift basis) makes it possible to control the ultrasonic beam angle and allow ultrasonic wave to focus. When receiving reflected ultrasonic waves, summing up these waves received by the piezoelectric elements on a time-shift basis on the time axis makes it possible to receive ultrasonic waves at one focal position in a similar way to transmission. The phased array method makes it possible to apply ultrasonic waves at high speed without moving the ultrasonic sensor and control the beam angle and focus depth position of the ultrasonic wave without replacing the ultrasonic sensor. Therefore, it can be said that the phased array method enables high-speed and high-accuracy testing. Generally known processes for the phased array method include the linear scanning process which linearly feeds piezoelectric elements and the sectorial scanning process which changes ultrasonic-wave transmit and receive directions in a fan-like form.
On the other hand, the synthetic aperture method transmits an ultrasonic wave so that it widely spreads out into an object under test, and receives a reflected ultrasonic signal from the inside of the object. The operating principle of the synthetic aperture method is that a defect serving as a sound source of the received reflected ultrasonic wave exists on a circular arc having the position of a piezoelectric element which transmitted and received an ultrasonic wave as a center and the propagation distance of the reflected ultrasonic wave as a radius. Based on this operating principle, the ultrasonic sensor transmits and receives ultrasonic waves while sequentially changing the position of a piezoelectric element. At each vibrator position, a received waveform is spread out in a circular arc form through computer operations. Then, intersections of these circular arcs focus at one position where a defect exists (an ultrasonic wave reflection source) thus allowing the defect position to be located and imaged. Actually, the synthetic aperture method performs high-resolution imaging through computer operations using the position of the ultrasonic sensor and the ultrasonic waveform signal at that position. Details of computer operations are discussed in Nonpatent Document 2.
In recent years, new sensors such as a matrix array transducer and a ring array transducer have been developed. The matrix array transducer is composed of an array of piezoelectric elements arranged in a matrix pattern inside an array ultrasonic sensor, and the ring array transducer is composed of an array of coaxially arranged piezoelectric elements (including arrangements in the circumferential direction). Further, apparatuses that can transmit and receive ultrasonic waves by using a number of piezoelectric elements have come into practical use. Thus, the inside of an object under test directly under the ultrasonic sensor can be three-dimensionally imaged without moving the ultrasonic sensor. With a known method for three-dimensionally imaging the inside of an object under test, a two-dimensional array ultrasonic sensor transmits an ultrasonic wave sequentially from each element and then receives a reflected ultrasonic wave with all elements and, at the same time, three-dimensional aperture synthetic processing is performed so as to superimpose received echoes (refer to, for example, Patent Document 1).
[Patent Document 1]
JP-2005-315582-A
[Nonpatent Document 2]
Michimasa Kondo, Yoshimasa Ohashi, and Akio Jitsumori, Digital Signal Processing Series Vol. 12, Digital Signal Processing in Measurement and Sensors, pp. 143-186, May 20, 1993, SHOKODO CO., LTD.
In recent years, new ultrasonic testing methods targeting various kinds of structural materials have been developed. As represented by the phased array method, these new techniques image and test the inside of an object under test with high accuracy in a short time (refer to, for example, Nonpatent Document 3).
The phased array method utilizes an array of a plurality of piezoelectric elements (also referred to as ultrasonic array transducer) and operates on a principle that wave fronts of ultrasonic waves transmitted from the piezoelectric elements mutually interfere to form one combined wave front in the course of propagation. Therefore, controlling the timing of ultrasonic wave transmission from each piezoelectric element with a time delay (on a time-shift basis) makes it possible to control the ultrasonic beam angle and allow the ultrasonic wave to focus.
When receiving reflected ultrasonic waves, summing up these waves received by the piezoelectric elements on a time-shift basis in accordance with the delay time makes it possible to control the receive beam angle of one combined ultrasonic wave as well as receive ultrasonic waves at one focal position in a similar way to transmission.
Generally known processes for the phased array method using a one-dimensional array transducer having linearly arranged piezoelectric elements include the linear scanning process which scans in ultrasonic-wave transmit and receive directions together, and the sectorial scanning process which changes ultrasonic-wave transmit and receive directions in a fan-like form centering on an incident point. Further, the use of a two-dimensional array transducer having piezoelectric elements arranged in a lattice pattern makes it possible to three-dimensionally focus on a desired spatial position, allowing selection of a scanning process which best suits the shape of the object under test. In particular, the three-dimensional scanning technique makes it possible to apply ultrasonic waves at high speed without moving the sensor, and control the beam angle and focal depth position of the ultrasonic wave, allowing high-speed and high-accuracy testing.
At present, in order to locate a spatial position of a reflection source from reflected ultrasonic wave signals, a method for presuming a spatial position from a plurality of two-dimensional images of reflection strength distributions at different cutting positions is commonly used (hereinafter this method is referred to as two-dimensional phased array method). For example, since the linear and sectorial scanning processes can obtain a plurality of two-dimensional images corresponding to a scanning range and interval, the direction in which a reflected wave appears can be located by sequentially changing the images on the display screen.
Recently, a new three-dimensional display method (hereinafter referred to as three-dimensional ultrasonic testing method) has been reported. This method performs interpolation processing to reflected ultrasonic wave signals from a plurality of directions to create three-dimensional lattice-like data and then performs volume rendering and surface rendering techniques to the created data. Although there are more than one method for creating three-dimensional lattice-like data, for example, the synthetic aperture method and phased array method, a method based on the phased array method is particularly referred to as three-dimensional phased array method (refer to, for example, Nonpatent Document 2). As three-dimensional lattice-like data, a data structure composed of a plurality of three-dimensionally arranged cubic elements (referred to as voxels) is most widely used because of ease of handling. This structure is also referred to as structural lattice. Although a lattice having irregular spatial lattice arrangements may be used in addition to voxels, such a lattice is slightly more difficult to display than a voxel. This kind of lattice is referred to as non-structural lattice as represented by a six-face lattice, a four-face lattice, a triangular pyramidal (prism) lattice, and a quadrangular pyramidal (pyramid) lattice. Further, there is another method for displaying data as three-dimensional point groups without conversion to lattice-like data. Since these pieces of data are saved in computer memory as three-dimensional testing data, they can be checked from any desired direction by an inspector after measurement.
In recent years, flaw size measurement (sizing) using the phased array method has attracted attention in industrial fields. Particularly in the field of nuclear power, the phased array method has been specified as a method for sizing a fatigue crack of carbon steel and stainless steel and a crack height of a stress corrosion crack (SCC) of stainless steel by technical guidance JEAC 4207-2004 of the Japan Electric Association which serves as an evaluation criterion for the soundness of domestic light-water nuclear reactors. At present, this guidance is taken over to technical regulation JEAG4207-2008 of the Japan Electric Association. The scope of the phased array method has been expanded not only as a method for sizing crack height but also as a method for checking the existence of a crack (refer to, for example, Nonpatent Document 4).
When measuring a flaw height (crack height), the two-dimensional phased array method utilizes sectorial-scanned or linear-scanned images including echoes at ends of a flaw. In this case, measurement must be performed according to defined measurement and analysis procedures, and it is recommended to validate the procedures by using a test piece having a flaw. These procedures are prescribed as flaw height measurement method based on the tip echo technique by NDIS 2418 standard of the Japanese Society for Non-destructive Inspection (refer to, for example, Nonpatent Document 5).
However, with the two-dimensional phased array method, echoes corresponding to upper and lower ends of a crack (hereinafter referred respectively to upper- and lower-end echoes) need to be included in the same screen. Therefore, it is necessary to finely adjust the sensor position and the ultrasonic beam angle depending on the orientation of a flaw. This method is time-consuming and requires experience to a certain extent. If the shape of the flaw is included in the same plane, it is preferable to find and measure an image in which upper- and lower-end echoes are clearly displayed in this way. However, if the shape of the flaw is complicated with many branches, such as scc, the shape of the flaw is not necessarily included in the same plane. In this case, two or more images are needed to measure the flaw height accuracy with the two-dimensional phased array method.
In this case, the use of the three-dimensional ultrasonic testing method is very effective. Although there are not many cases reported, a sizing method based on the three-dimensional ultrasonic testing method has been devised. A method discussed in Nonpatent Document 6 displays measurement data points obtained by a plurality of tests on a screen as point groups. With a desired cross section displayed, for example, when two points corresponding to upper- and lower-end echoes are specified by using a mouse or keyboard of a computer, the distance between the two points is output. With the two-dimensional phased array method, it is necessary to find a screen in which upper- and lower-end echoes are simultaneously included at the time of data storage. With the three-dimensional ultrasonic testing method, on the other hand, it is only necessary to perform a series of data storage for a predetermined testing range and then find a target cross section. The latter method makes testing procedures very efficient and is advantageous.
[Nonpatent Document 3]
Yoshikazu Yokono, Global Trend of Phased Array Ultrasonic Testing Its Practical Application and Standardization, The Japanese Society for Non-destructive Inspection, Vol. 56, No. 10, (2007)
[Nonpatent Document 4]
Atsushi Baba, Satoshi Kitazawa, Naoyuki Kono, Yuji Adachi, Mitsuru Odakura, and Osamu Kikuchi, Development of Three-dimensional Ultrasonic Testing System 3D Focus-UT, JAPAN SOCIETY OF MAINTENOLOGY, 5th Academic Lecture, Collection of Summaries, 155 (2008)
[Nonpatent Document 5]
The Japanese Society for Non-destructive Inspection NDIS 2418:2005, pp. 21
[Nonpatent Document 6]
Potts, A.; McNab, A.; Reilly, D.; Toft, M., “Presentation and analysis enhancements of the NDT Workbench a software package for ultrasonic NDT data”, REVIEW OF PROGRESS IN QUANTITATIVE NONDESTRUCTIVE EVALUATION: Volume 19. AIP Conference Proceedings, Volume 509, pp. 741-748 (2000).